Relay and Neutral safety switch Question.

The starter coil has an upstream wire (yellow) from the key and a downstream wire (usually violet) to the NSS. Breaking either with a push-button switch will disable the relay. The upstream wire is already inside the cabin, so easier wiring.

As mentioned, any released prisoner would know the "jumper starter w/ screwdriver" trick. Thus, perhaps better to disable the ignition system. That can be done more subtly with electronic ignition. If you have points, most thieves would know how to jumper the battery to coil+ (though not all thieves carry alligator leads), so better to break the path from coil- to points. Whatever approach, wire a toggle switch in parallel w/ the push-button so you can over-ride for valets and such.